Roll 50: Copied IDLs, PYTHON scripts from WebKit removed deleted files in WebCore

core/events/AnimationPlayerEvent.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // http://www.w3.org/TR/web-animations/#the-animationplayerevent-interface
 
 // TODO(dstockwell): This interface has been removed in the latest spec:
 // https://w3c.github.io/web-animations/
 
 [
     Constructor(DOMString type, optional AnimationPlayerEventInit eventInitDict),
     RuntimeEnabled=WebAnimationsAPI,
 ] interface AnimationPlayerEvent : Event {
-    readonly attribute double currentTime;
+    readonly attribute double? currentTime;
     readonly attribute double timelineTime;
 };